Look at the three cells marked with asterisks. The only candidates that can fit are 4, 7 and 9 in some order. Combine that with the 7 at r5c1 and the 7 in r7c2 -- only one cell remains in box 1 that can hold a 7.

For some, it's easier to see it the other way around -- there's a 1 and a 5 already placed in columns 4, 7 and 9. That leaves only two cells in row 3 that can hold a 1 or a 5. Since r3c3 and r3c6 must hold a 1 and a 5 in some order, they *can't* hold a 7 -- and again, only one cell remains in box 1 that can hold a 7.